Fixed standard materials shaders to use properly transformed positions and normals with skinning and morphing.

Also while at it I did some shader snippets cleanup and optimizations here and there.

There were quite drastic changes, impacting almost everything. It should work, I went through all WebGL examples, but you never know, there are myriads of possible material combinations. Please let me know if you find something broken.

Instead of adding the helper as a child of the camera, the helper now steals the matrixWorld of the camera.
This allows for having the helper in another scene of where the camera is (GUI).

See #2336

See #2336

Now it doesn't throw exception but still doesn't work ok, there are freaky artefacts.

Fixed bug in CanvasRenderer where it would try to render a smooth shaded face even if mesh had no vertex normals.

Removed skinVerticesA/B arrays.

Changed 'text/plain' -> 'application/json' in JSONLoader.js (it solved some issues with large files loading)